[in 1960 at MIT, “Mash Until No Good”; sometime after
that the derivation from the recursive acronym
“Mung Until No Good” became standard; but see
munge]

1. To make changes to a file, esp. large-scale and irrevocable
changes. See BLT.

2. To destroy, usually accidentally, occasionally maliciously. The
system only mungs things maliciously; this is a consequence of
Finagle's Law. See scribble,
mangle, trash,
nuke. Reports from Usenet
suggest that the pronunciation /muhnj/ is now usual in speech, but the
spelling ‘mung’ is still common in program comments (compare
the widespread confusion over the proper spelling of
kluge).

3. In the wake of the spam epidemics of the
1990s, mung is now commonly used to describe the act of modifying an email
address in a sig block in a way that human beings can readily reverse but
that will fool an address harvester. Example:
johnNOSPAMsmith@isp.net.

4. The kind of beans the sprouts of which are used in Chinese food.
(That's their real name! Mung beans! Really!)

Like many early hacker terms, this one seems to have originated at
TMRC; it was already in use there in 1958. Peter
Samson (compiler of the original TMRC lexicon) thinks it may originally
have been onomatopoeic for the sound of a relay spring (contact) being
twanged. However, it is known that during the World Wars,
‘mung’ was U.S.: army slang for the ersatz creamed chipped beef
better known as ‘SOS’, and it seems quite likely that the word
in fact goes back to Scots-dialect munge.

Charles Mackay's 1874 book Lost Beauties of the English
Language defined “mung” as follows:
“Preterite of ming, to ming or mingle; when the substantive meaning
of mingled food of bread, potatoes, etc. thrown to poultry. In America,
‘mung news’ is a common expression applied to false news, but
probably having its derivation from mingled (or mung) news, in which the
true and the false are so mixed up together that it is impossible to
distinguish one from another.”